1. Field of the Invention
This invention generally relates to methods for fabricating microvalves. More specifically, the invention relates to procedures for forming a contoured valve stop in a microvalve.
2. Background Art
Microvalves may be used to control the flow of a gas in high temperature, high corrosive environments. In one such microvalve, a thin diaphragm, sandwiched between adjacent SiC or Si layers or wafers, is used to control the flow of gas through the valve. In use, the diaphragm abuts against a seat around an inlet of the valve to close that inlet; and when the pressure of gas in this inlet rises above a given value, the diaphragm flexes upwards, allowing gas flow through the inlet and through the valve.
A cavity or recess is provided inside the valve to allow this upward flexing movement of the diaphragm; and as the diaphragm flexes upward, the diaphragm comes into contact with surfaces of this cavity. This contact may cause significant localized forces or stresses on the diaphragm, or specific portions of the diaphragm.
Shaping this cavity or recess in any particular way is difficult because of the very small dimensions involved. For instance, this cavity may have a width or diameter of about one millimeter and a depth of about 0.025 millimeters. In the past, reactive ion etch (RIE) has been used to create very small, three-dimensional features in SiC and/or Si wafers. RIE etches can provide excellent trenches with steep smooth side walls, but RIE does not lend itself to creating contoured shapes. Since RIE uses a metal mask, it precludes using a gray scale mask. In addition, RIE is time consuming (etch rate of 0.001 mm/min). Inductively coupled plasma (ICP) etch techniques can also make clean well-defined trenches, but these techniques also are not conducive to making sloped and contoured surfaces. Lastly, the microvalve features are much too small for mechanical machining.